U.S. Pat. Nos. 5,620,744, 5,683,748, 5,718,944, 5,834,075 and 6,056,997 outline the basic causes of corrosion in sewer pipes, such as concrete, iron or steel, as well as the prior attempts to inhibit such corrosion and to reduce the damage and ultimate failure collapse and failure of sewer pipes.
The problem has been around for decades and is not going away.
The proposed solutions have ranged from (1) crown spaying or otherwise coating of the inner surfaces of sewer pipes with lime and/or caustic soda slurries as described in the U.S. Pat. Nos. 5,620,744 and 5,683,748 were the purpose of the crown spray process is to leave residual alkalinity on the sewer crown, to (2) the currently exclusively used slurries of 50 to 60% by volume of magnesium hydroxide alone in 40 to 50% water mixture as described in the U.S. Pat. Nos. 5,718,944, 5,834,075 and 6,056,997. Unfortunately, sewer pipes continue to corrode and fail.
The cause of the problem is well understood. As shown in the following FIG. 1, sulfate ions occur naturally in most water supplies and are present in sewage as well, sulfur being required for the synthesis of proteins and being released during their degradation. In a moist atmosphere, such as the anaerobic atmosphere found in sewer pipes, those sulfate ions are chemically reduced to hydrogen sulfide (H2S) and biologically oxidized by bacteria called thiobacillus to sulfuric acid (H2SO4), which is highly corrosive to sewer pipes made of materials soluble to sulfuric acid, such as concrete, iron or steel.
Applicant's solution to the problem of sewer pipe corrosion is quite simple. When applied to the current methods of coating the interiors of sewer pipes using the well known crown spay procedure it simply involves the addition of less than 1% of sodium hydroxide by volume to the currently applied slurry of 50 to 60% by volume of magnesium hydroxide alone. The result is an immediate increase of the pH of the slurry from about 8.5 to 8.8 to a pH of 13 or above accompanied by an increased useful life of the slurry from its current 6 to 8 months to 16 to 18 months, that is an improvement of over 100%.
Similar improvements are associated with other of Applicant's proposed solutions as described in detail hereinafter, namely slurries of titanium dioxide and less than 1% by volume sodium hydroxide and slurries of magnesium hydroxide, titanium dioxide and less than 1% by volume sodium hydroxide were the useful lives of the slurries are 20 to 24 months and 16 to 18 months respectively which are improvements of over 200% and over 100% when compared to the currently used slurry of magnesium hydroxide alone.